A patient with a history of two THR on the same side due "loosening" as per patient and the last surgery was one year ago. If during the eval the hip is lmited to 0 deg of IR. Should one attempt to gradually improve IR to 10-15 deg

Yes you can gradually progress them up to 45 degrees of hip IR or whatever you can eventually stretch them out to. Progress should be in small incremental degrees until an end range is reached. Since it's been a year since the last THR surgery, then it is well beyond the protection phase for the prosthesis.